The emerging markets of electric vehicles (EV) and plug-in hybrid electric vehicles (PHEV) generate a tremendous demand for low-cost lithium-ion batteries (LIBs) with high energy and power densities, and long cycling life. Anode materials in commercial LIBs are primarily synthetic graphite-based materials with a capacity of ˜370 mAh/g. Improvements in anode performance, particularly in anode capacity, are desirable to achieving higher energy densities in LIBs for the EV and PHEV applications.
Silicon has been pursued as a promising anode material for high-energy-density LIBs because of its high specific capacity (>3500 mAh/g) and abundance. Despite its high capacity, silicon (“Si”) suffers from the fast capacity fading caused by large volume change (>300%) and the resultant loss of electric contact and disintegration (cracking and crumbling) of the anode structure during lithiation and delithiation.
The development of silicon-carbon (“Si—C”) nanocomposites, composed of -silicon nanostructures (e.g., nanowires, nanotubes, nanoparticles) intimately contacted with carbon, has been widely studied. These nanocomposites proved as an effective solution to improve capacity cycling stability as nano-sized silicon can alleviate physical strains and mechanical fracture generated during volume changes to prevent the fast disintegration. Intimated contact between silicon and carbon can maintain anode structure integration.
The use of nano-sized silicon materials presents difficulties in practical application as electrode materials for LIBs. First, nano-sized materials may pose health risks to humans and harmful effects to the environment. They may have other safety issues, which may hamper their application to a great extent. Second, a high tap density is an important factor, especially for fabrication of high-energy LIBs for EVs and HEVs because it offers low reactivity and a high volumetric energy density. Unfortunately, tap density of nano-sized materials is generally low, which in turn holds down their specific volumetric capacity. Furthermore, preparation for the nano-sized silicon either requires chemical/physical vapor deposition or involves complicated processes, which has drawbacks of high cost and low yield, and is difficult to scale up. To date, the advantage of abundance of silicon has not been fully embodied due to lack of low-cost strategy for large-scale synthesis of silicon anode materials with superior performance.
Micro-sized materials are favorable for practical battery application since they assure higher tap density than nano-sized materials and, as a result, are expected to offer higher volumetric capacity. However, the limitations of micro-sized silicon materials are clear. The solid micro-sized silicon materials are more likely to undergo mechanical disintegration upon volume change during lithiation and delithiation compared with nano-sized materials, resulting in severe capacity fading. The micro-sized materials also have long ion and electron transportation paths, which adversely affect high rate capability. From this point of view, it is desirable to develop new materials that combine the advantages of both micro-sized and nano-sized silicon materials to improve cycling performance and energy density of silicon anodes.
Two models of micro-sized silicon anode materials with nanostructured building blocks have been proposed. One model is a porous silicon material in which the volume change of the silicon can be accommodated by pores to improve cycling stability. Cho et. al. report a porous silicon composed of a nanocrystalline silicon framework created by a templated approach converting silicon gel to porous silicon. The porous silicon shows a superior performance with high capacity and good cycling stability. The synthesis involves complicated synthesis of silicon gel obtained from reduction of SiCl4. This leads to low yield and high cost of the silicon anode.
A low cost approach to produce porous micro-sized silicon has been reported by catalytically etching surface layers of micro-sized bulk silicon, and the obtained porous micro-sized silicon powder shows good cycling stability within only limited cycles (not more than 70 cycles). Another example is micro-sized C—Si nanocomposite spherical granules prepared by physically depositing silicon nanoparticles into porous micro-sized carbon granules, which exhibited a good capacity retention of about 1500 mAh/g after 100 cycles at 1 C, a tap density of 0.49 g/cm3 and a volumetric capacity of approximately 1270 mAh/cm3 at C/20. Solid micro-sized composites consisting of interwoven nano-components is the other model that successfully shows superior cycling performance and high energy density. The volume change of active materials is buffered by the surrounding integrated components. For example, carbon-coated micro-sized silicon-based multicomponent anodes consisting of Si/SiO cores and crystalline SiO2 shells show a high reversible capacity of 1280 mAh/g after 200 cycles and a volumetric energy density of 1160 mAh/cm3.
In spite of microscale sizes as a whole, such micro-sized Si—C nanocomposite materials not only fully utilize the advantages of their nano-sized silicon building blocks but also integrate them to micro-size bulk form to reach a high volumetric energy density. However, reports on synthesis of micro-sized Si—C nanocomposite anode with good cycling performances are still limited.